For me, it was Spine. imho - the concept of that fight is better than any other I've done. So, I had such a fun time doing it.
Prior to that, the fight that sticks out most in my head is Sarth 3D. When I first tried it (before we tried 1D or 2D), it blew my mind away. The complete chaos reminded me of the first few times we tried HKM.
Will of the Emperor easily, at least as tank. They really should have forced the entire raid into the range of Devastating Combo during Titan Gas though, that would have made the encounter perfect.